The invention relates to apparatus for monitoring the liquid level in a tank with a resistance bridge supplied with d-c current in general, and more particularly to an improved arrangement to prevent disturbances when testing such apparatus.
Apparatus of this general nature, also called an electrical level sensor, is known from "Siemens-Zeitschrift" 42 (1968), pages 746 to 748. With such level sensors, the water level in steam boilers and condensers of steam power plants is monitored and a warning signal is given if the level exceeds or falls below a predetermined value. The output voltage of the resistance bridge with two arms inside the tank, one of which is heated, is used to provide the warning signal. The fact that the bridge arms if immersed in the liquid, e.g., water, will be at approximately the same temperature even though the one bridge arm is heated, while in a steam atmosphere the bridge arm inside the tank will have different temperatures because of the heating is used as a basis for evaluation. In the latter state, the bridge is therefore unbalanced. Where water is the cooling liquid, the fact that, with increasing pressure, the density difference and, therefore, the temperature difference between steam and water becomes smaller and smaller until it becomes zero at the critical pressure must also be considered. This also means that the heat transfer between steam and water is equal at the critical pressure. In order to compensate for this decrease of the temperature difference with increasing pressure, the bridge output voltage must therefore be regulated to a constant value by means of the current regulator. If both bridge arms are in the steam atmosphere the heating current must be limited in order to avoid overheating if the density of the steam atmosphere is too low. In order to test the operability of the level sensor during operation, a testing arrangement is provided in which the bridge is unbalanced by shunting resistors across it and thereby simulating a given filling level. In the prior art apparatus the length of the connecting line between the resistance bridge and the evaluating device influences the measurement value, so that an adjustment at the installation site is always necessary. In addition, the testing arrangement does not determine whether the sensor formed with the resistors is functioning properly. With the prior art device, difficulties are therefore encountered not only in the installation, but also because it does not always operate in a fully reliable manner.
Thus, there is a need for improved apparatus of the general type mentioned above which is easy to install and which has full operational reliability. pg,3